gig PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxv. 



as a Bollia by Ulrich, II. radiata (Beyrichia radiata Jones and 

 Kirkby), variety cestriensis Ulrich, II. longispina {Beyrichia longis- 

 p'nia Jones and Kirkby), and II. emaciata (Beyrichia f emaciata 

 Ulrich and Bassler). 



Comparison of the figures on Plate XLII gives a good idea of the 

 unusual range of variability of the species associated in this new 

 genus. On closer study, however, strong elements of similarity will 

 be noted running through the whole assemblage, the observer being 

 finally convinced of the essential naturalness of the association. 

 It is not contended that the group is natural in the sense of being 

 composed of species descended from a single ancestor. On the con- 

 trary, it is believed they were derived from perhaps several preced- 

 ing species of Beyrichia and possibly Ctenobolbina, and that the 

 singularities resulted through atavistic tendencies developed in the 

 Devonian decadence of the Silurian Beyrichiidae. 



The Devonian species of the genus may be regarded as vacillating 

 intermediate stages between the trilobate Silurian Beyrichia and the 

 final, again long-lived, simply binodate Carboniferous phase of the 

 new genus, namely, the small group of species of which //. radiata 

 (Jones and Kirkby) is a typical example. Whereas most of the 

 Devonian species occur at the Falls of the Ohio in a thin Wed thought 

 to be of Onondaga age, indicating rapid evolution. II. radiata ranges 

 with very slight change from the Chester to near the close of the 

 Pennsylvanian. 



Compared with true Beyrichia, which has constantly three verti- 

 cally elongated lobes, the new genus Hollina is distinguished (1) by 

 the progressive obsolescence of the posterior lobe, (2) by the' rounded 

 form of the median and anterior nodes or lobes. (3) by the restric- 

 tion of these nodes to the dorsal half of the valve. (4) by the rela- 

 tive fullness of the ventral parts (agreeing in this respect with 

 Klcedenia), and (5) by the constant development of an anteriorly 

 incomplete marginal frill. A probable sixth difference is indicated 

 by the apparent absence of a ventral pouch. 



The fullness of the ventral region, also the rounded form of the 

 median (posterior) node suggests even closer alliances with 

 Klwdenia, but the rather obvious relations of Hollina to Treposella 

 lyoni and the probable derivation of that species from Beyrichia 

 tends to negative this suggestion. The Devonian species would never 

 cause one to think of Klwdenia. It is only the more simple //. 

 radiata group that might recall that Silurian genus,, But these even 

 will be distinguished at once by the«anterior node, the like of which 

 never occurs in Kla <l< nia. 



In a former paper," the writers mention the possible desirability 

 of including B. radiata and its immediate allies in Ulrichia. It is 



°Proc. r. S. Nat. Mus., XXX. 1906, p. 152. 



